Spoken word recognition is at the root of language comprehension and acquisition. For adults, recognition of known words (numbering in the tens of thousands) is fast, accurate, robust, and effortless. Yet the perceptual computations required are highly complex, as anyone listening to speakers conversing in an unfamiliar language begins to discern: what one hears is a babble lacking readily identifiable words. Nevertheless, within the first year of life, infants must develop the skills needed for spoken word recognition. Such skills are prerequisite to further acquisition of language, for if a language learner cannot break input utterances into their constituent words, it will be impossible to learn how these words fit together or what individual words mean. Studies proposed in this application address acquisition of the three chief components of spoken word recognition: segmentation of continuous speech into word-like chunks, representation of speech in a compact fashion that maximizes efficiency of processing, and identification of tokens as exemplars of particular lexical types. Issues considered include: How does developing lexical knowledge affect segmentation? How are knowledge-driven and signal-driven segmentation related? What are the constraints on properties of speech that infants initially entertain as contributing to lexical representations? The range of such properties narrows with development: what are the statistical properties of input that underlie this change and guide the emergence of native-language phonological categories? Adult word recognition is affected by number and frequency of competitors. How are competitive neighborhoods structured in the developing lexicon? How do processing deadlines and perceptual characteristics of speech tokens interact in word identification? The proposed research will employ a variety of empirical methods to provide converging measures of the development of spoken word recognition in infants from 6 to 18 months: headturn preference, conditioned head-turning, habituation, and intermodal preferential looking. Results from experimental studies with infants will be integrated with computational modeling to assess the feasibility of continuity theories of word recognition development, according to which fundamental processing architecture is shared by infants and adults.